The present invention generally relates to digital pre-distortion adaptation techniques for cellular base station power amplifiers.
Today's cellular base stations must be operated efficiently to minimize power dissipation and reduce the need for costly cooling equipment. Digital pre-distortion (DPD) plays a key role in correcting power amplifier (PA) nonlinear behavior and enabling PAs to operate with the high efficiency and linearity necessary to meet stringent adjacent channel leakage ratio (ACLR) specifications. Adaptive nonlinear systems that are periodically trained using snippets of the actively transmitted signal are very sensitive to the signal samples selected for adaptation (training). Digital pre-distortion for power amplifier linearization is one such system.
There are several conventional methods that select signal snippets for adaptation. These are conventionally based on the peak content in the signal and the average (RMS) power level. However, for systems with significant nonlinear memory, these metrics alone often come up short in being able to provide optimal performance that is stable over time.
Frequency hopping is used in multicarrier wireless systems, to improve the signal to noise ratio in a link by adding frequency diversity. Signal fading can occur when the signal from a wireless system base station reflects from multiple surfaces, e.g. buildings, vehicles, such that the multiple reflected signals arrive at the same receiving point having different path lengths and therefore different phases. These multiple reflected signals interfere with each other and, depending on the phase relationships between the signals, the addition of the out-of-phase multiple signals can cause both constructive and destructive interference. When destructive interference occurs, signal amplitude is reduced and thus signal fading occurs. Frequency diversity, i.e. changing the carrier frequency, combats such fading since it also changes the phase relationships between the reflected signals. In multicarrier wireless systems, the frequencies of the carriers are constantly exchanged with each other over time in a repeating pattern. This is known as “frequency hopping” and the repeating pattern is known as a “hopping pattern”. In addition, the hopping patterns themselves are changed over time. Carrier frequency changes and pattern changes are done synchronously on pre-determined time boundaries.
Out of all applied cellular standards, multicarrier wireless systems require the most stringent ACLR regulatory requirements as compared to typical 3G or 4G systems. How to design DPD components for multicarrier wireless hopping systems has been one of the most challenging problems faced by the industry in the recent past. DPD performance in multicarrier wireless hopping systems is very often used as a metric to evaluate different DPD solutions for use across different platforms.
The conventional method for choosing snippets or captures of signal for DPD adaptation, as mentioned, is to base the choice on the peak and RMS power content. This can be further explained with a diagram.
FIG. 1 shows a system 100 of a conventional DPD adaptation system using peak and RMS power thresholds for choosing signal captures for training.
System 100 includes DPD component 104, a transmitter/PA component 113, a signal tap component 117, a capture filter component 118 and an adaptation component 134. Transmitter/PA component 113 includes a transmitter 108 and a PA 112. Signal tap component 117 includes a coupler 120 and a receiver 124. Capture filter component 118 further includes a comparator 128 and a storage component 130.
DPD component 104 is arranged to input a signal on a line 102 for transmission and to output a signal on a line 106 to transmitter 108. Transmitter 108 is arranged to connect to PA 112, via line 110. PA 112 is arranged to connect to antenna 116 via a line 114, which also passes through coupler 120. Receiver 124 is arranged to input a signal on a line 122 from coupler 120 and connects to comparator 128 via a line 126. Storage component 130 connects to capture component 128 via a line 129. Comparator 128 is arranged to output a signal on a line 132 to adaptation component 134. Adaptation component 134 outputs a signal on a line 136 to DPD component 104.
DPD component 104 is operable to pre-distort an input signal. Transmitter 108 is operable to provide digital-to-analog (D/A) conversion and frequency up-conversion functions. PA 112 is operable to provide RF signal amplification. Antenna 116 is operable to transmit an RF signal over the air. Coupler 120 is operable to tap off a small portion of the power of an input signal and to pass through the remainder of the power. Receiver 124 is operable to provide down-conversion and analog-to-digital (A/D) conversion functions. Comparator 128 is operable to capture a snippet of an input signal in time, to compare properties of the capture against predetermined thresholds and to filter out undesired captures. Storage component 130 is operable to store and provide signal property threshold values. Adaptation component 134 is operable provide DPD adaptation signals based on signal captures at its input and to provide adaptation timing and synchronization.
DPD component 104, transmitter 108, PA 112 and antenna 116 form the transmit chain for a cellular base station radio. Line 102 carries the digital signal input intended for transmission via the transmit chain. Transmitter 108 converts the digital signal into an analog signal and up-converts the analog signal to radio frequencies (RF) on line 110. The upconverted signal is then sent to PA 112 for amplification before it is transmitted over the air via antenna 116. However, since PA 112 is operated in its most efficient region i.e. the non-linear high gain region, its input must be conditioned in order to compensate for the non-linearities of the PA's gain response. This compensation is performed by DPD component 104. DPD component 104 modifies the input signals on line 102 to produce compensated signals on line 106. The compensation is such that when a signal on line 110, which is the D/A converted and up-converted version of signal 106, passes through PA 112 in its nonlinear region, the result approximates the undistorted signal passing through a PA with a linear gain response.
The pre-distortion applied by DPD component 104 needs to be adapted over time due to changes in the input signal properties and behavior changes over time in the analog system, especially PA. To accomplish this, a portion of the transmitted signal is captured and its properties used for the adaptation. Coupler 120 taps a portion of the power of the signal 114 and this is fed through receiver 124 which applies frequency down-conversion and A/D conversion to produce signal 126, which, in effect, for the tapped off signal, “undoes” the D/A conversion and upconversion performed by transmitter/PA 113. Therefore, signal 126 can be considered a replica of signal 106. Comparator 128 captures and stores a slice in time, or snippet, of signal 126, then determines the peak and average RMS values within the snippet. The peak and average RMS values in the snippet, being representative of those of signal 106, can then be used as properties to perform DPD adaptation.
Empirical testing has shown that not all captures can yield good results when used for adaptation and that captures need to be individually selected for this purpose. In this conventional system, captures are selected on the basis of the peak and average RMS powers contained in the capture and the threshold values of these properties which have been determined as the criteria for selection have themselves been determined empirically.
In operation, storage component 130 holds the peak and RMS thresholds used and supplies the values to comparator 128 which then, by comparing them to the values obtained from the snippet via signal 126, uses them to determine the suitability of the capture for adaptation use. Comparator 128 then filters out those which do not meet the thresholds and so have been found not suitable. The remaining captures are passed to adaptation component 134 which determines the changes needed to the adaptation method and communicates these changes to DPD component 104 via signal 136.
For systems, especially those with significant nonlinear memory, these metrics alone (peak and RMS power) can often come up short in being able to provide optimal performance which is stable over time.
In frequency hopping systems such as multicarrier wireless systems, DPD adaptation to the different hopping patterns can also present problems. Existing solutions can use a synchronous memory and processing-intensive method or use fast re-adaptation along with PA back-off, a method which offers significantly lower power efficiency. The conventional method to overcome such problems is known as the “brute force” method, whereby a different DPD solution for each hopping pattern has to be applied each time there is a new hopping pattern transmitted.
FIG. 2 shows a block diagram 200 which illustrates the treatment of different hopping patterns by the “brute force” method.
In the figure, block diagram 200 includes system 100, a system 202 and a system 204. System 100, as described for FIG. 1, includes DPD component 104, transmitter/PA 113, signal tap component 117, capture filter component 118, and adaptation component 134. Both system 202 and system 204 include the components of system 100. System 202 and system 204 represent system 100 at a later time in a frequency hopping pattern sequence.
Line 102, line 106, line 110, line 114, line 132 and line 136 of system 100, system 202 and system 204 are arranged in the same manner as for system 100 of FIG. 1.
System 100 is operable exactly as described for system 100 in FIG. 1. System 202 and system 204 are both identically operable as system 100.
As described earlier, in multicarrier wireless systems, frequency hopping is the practice of exchanging the frequencies of the multiple carriers with each other over time in a repeating pattern and that the hopping patterns themselves are changed over time. For example, a wireless channel may be assigned four carrier frequencies f1, f2, f3 and f4 will change between those frequencies in order on pre-determined time boundaries and the pattern will repeat constantly. At another time, the channel may have the hopping pattern f5, f6, f7 and f8 will behave in the same manner. It should be noted that the frequency pattern is chosen such that the same frequency is not being used twice at any one time. In addition, the patterns themselves can change over time such that there is a repeating sequence of patterns. For example, the channel may change from pattern f1, f2, f3, f4 to pattern f5, f6, f7, f8, then to f1, f3, f5, f7 then to f2, f4, f6, f8. There may also be additional patterns. The pattern changes, as for the frequency changes, must be done synchronously on pre-determined time boundaries.
Referring again to the figure, system 100, system 202 and system 204, while identically operable, differ in that each is configured to handle a different frequency hopping pattern from a sequence of N patterns. System 100 is configured to handle a hopping pattern 206, the first hopping pattern, system 212 is configured to handle a hopping pattern 208, the second hopping pattern and system 222 is configured to handle a hopping pattern 210, the Nth hopping pattern.
From the figure, at time t1 for system 100, DPD adaptation to the properties of hopping pattern 206 starts at the time that a hopping pattern change occurs and hopping pattern 206 begins. Sometime later, at time t2, the hopping pattern changes from hopping pattern 206 to hopping pattern 208 and DPD adaptation also changes, illustrated by system 202. DPD adaptation changes then occur for every hopping pattern change until, at time t3, the hopping pattern changes to hopping pattern 208, the Nth hopping pattern, illustrated by system 204. The hopping sequence itself then starts repeating.
It should be noted that in cellular conventional systems hopping pattern changes are done with reference to available system timing signal boundaries, such as numbered frame or superframe boundaries. In addition, channel frequency assignment information which is necessary to implement frequency hopping changes is made available to all the components which require it.
Each time there is a hopping pattern change, a new method must be loaded into DPD component 104 and feedback processor 113 must be loaded with new thresholds as directed by adaptation component 134. Such changes must occur instantaneously, necessitating preloading of methods and thresholds into ready-to-go buffers. Changes must also occur synchronously requiring timing circuitry within timing component 134. Conventional solutions for multicarrier wireless frequency hopping solutions therefore can require relatively heavy resources for processing, storage and timing.
One problem with conventional DPD adaptation techniques is that the results can often fall far short of optimal performance. Additionally, in multicarrier wireless frequency hopping applications using conventional DPD adaptation techniques, the processing, memory and timing circuitry required to implement conventional brute-force techniques can be very intensive.
What is needed is a system and method which can evaluate signal captures for their suitability for DPD adaptation which eliminates many of the performance problems of conventional systems and methods. What is also needed, when frequency hopping is present, is a system and method which would eliminate much of the hardware required by conventional systems and methods.